Two new studies add to the confusion over the health effects of marijuana.

In one presentation to the American Stroke Association’s International Stroke Conference, researchers from the University of Auckland in New Zealand found an increased risk of stroke among those smokingmarijuana compared to those who did not, while research published in the American Heart Journal said marijuana users who had heart attacks were no more likely to die than those those hadn’t smoked cannabis.

The stroke study, which incorporated preliminary data, is the first trial of its kind to study a possible connection between marijuana use and stroke. It included 160 patients aged 18 to 55 who had suffered a stroke connected to a blood clot in the brain, and who agreed to have their urine tested for marijuana within 72 hours of the stroke. These results were compared to those from 160 controls who had not had a stroke but came to the hospital for other reasons. They were matched on age, gender and ethnic background, all of which can also affect the risk for this type of stroke. About 16% of the stroke patients showed traces of marijuana in their urine, compared to 8% of those in the control group, suggesting a doubling of the risk of stroke.

“This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis,” lead author Dr. Alan Barber of the University of Auckland in New Zealand said in a statement.

The study, however, could not separate tobacco smokers from marijuana smokers, because all but one of those testing positive for marijuana in the urine also showed signs of nicotine. Still, Barber said to Everyday Health “We know cannabis can cause changes in blood pressure and heart rate that are associated with increased stroke risk. Importantly, it can also cause heart palpitations, [a sign of atrial fibrillation]. And atrial fibrillation is very strongly associated with stroke.”

The heart attack study, led by Dr. Murray Mittleman of Harvard Medical School, followed nearly 4,000 heart attack survivors for up to 18 years to investigate any patterns between marijuana use and heart attack patterns. Among the participants, 109 had smoked marijuana at least once in the year before they were hospitalized. Most were in their 40s and 50s; none of those over age 63 had used marijuana.

During the study period, 519 patients died, including 22 marijuana users. But while there was an apparent 29% increase in mortality among those who used marijuana, the result was not statistically significant, meaning that it could have occurred by chance alone.

The study also did not find a relationship between the amount of cannabis used and the risk of death. While the researchers documented that any use was associated with more risk than no use, those who smoked more than once a week paradoxically seemed to be at less risk than those who smoked less than weekly.

Earlier research, however, found that the hour after smoking marijuana is associated with a two-fold to five-fold increase in the risk of heart attack, roughly the same risk seen within an hour of sexual activity. The authors conclude, “[I]t seems prudent to caution patients with coronary heart disease and those at high risk for cardiovascular disease to abstain from smoking marijuana.”

Taken together, the findings highlight the still-confusing state of marijuana research. While some studies have attempted to connect cannabis use with either short or long term health effects, it has proved extremely difficult to pinpoint the specific effects of marijuana because the heaviest marijuana users so often use multiple drugs. That’s in contrast to the research on tobacco, which is far more consistent. Cigarette smoking, for example, increases the risk of lung cancer by a factor of 23 for men and 13 for women, boosts stroke risk by up to four-fold and coronary heart disease by the same amount. More research using sophisticated controls is necessary to truly understand the risks, whatever they may be.